Electric power system



Jan- 29, 1946- M, A. EDWARDS ETAL 2,393,618

ELECTRIC OWERl SYSTEM Filed'nec. 19, 1942 2 Sheets-Sheet l Inventow-sr Mar-tin A. Edwar- Donald E. GaTT,

'T- AttoT-heg y .John C. Agdelott, b5 j/ CJMMM y Th2.

Jan. 29, 1946. M A. EDWARDS TAL 2,393,618

ELECTRIC POWER SYSTEM John C. A gdelott,

v,Theiv- Attorney. l

Patented dan. 29, 1946 ELECTRIC POWER SYSTEM Martin A. Edwards, Scotia, and Donald E. Gan', Schenectady, N. Y., and John C. Aydelott, Erie, Pa., asslgnors to General Electric Company, a corporation of New York Application December 19, 1942, Serial No. 469,538

54 Claims.

Our invention relates to electric power systemsy and particularly to gasor Diesel-electric power systems for driving self-propelled vehicles.

Diesel-electric power systems for self-propelled vehicles ordinarily comprise a prime mover, such as a Diesel or other internal combustion engine, arranged to drive a generator from which power is supplied to one or more traction motors. The systems of this type which are in more or less common use at the present time make use of traction motors of the series field type and rely upon voltage control of the generator for controlling the torque and speed of ythe motors, and reversing switches for controlling their direction of rotation. While such a system is suitable for application to vehicles in which all motors simultaneously undergo like changes of speed and torque, such as in Diesel-electric locomotives,

buses, and the like, they are not suiiciently flexible to supply the demands of certain other types of vehicles in which an optimum of maneuverability is desired. This is especially true of such vehicles as tractors, electric shovels, military tanks, and the like, which rely upon the independent control of laterally spaced driving elements for propulsion, steering, and braking. Essentially the same problems of propulsion, steering, and braking are encountered in twinscrew or paddle-wheel boats, although the difculty in these applications is minimized to some extent insofar as extreme rapidity of response, while it may be provided, is ordinarily not necessary.

For applications of the above nature, controlled current systems have been proposed. By such a system we mean one in which the main generator is connected in a loop circuit with the motor armatures and arranged to supply to the armatures a constant or otherwise continuously controlled unidirectional current, while the excitation of the motor field windings is separately and independently controlled to determine the speed, torque, and direction of rotation of each of the motors. Such a system is shown in broad outline in British Patent 226,960, and satisfactorily meets the principal demands of maneuverability outlined above. For example, it will be evident that both motors may be suddenly reversed to eiect dynamic braking merely by reversing the relatively small energizing current o! their neld windings, while the unidirectional armature current' is maintained constant or within predetermined limits. Similarly, the torque of any one motor may be reduced or,

in fact, reversed to effect steeringn It is also evident that an inherent advantage of this arrangement is that if during steering, one of the motors is exerting a reverse torque, or in other words, is being braked and acting as a generator, the power output of the'generating motor is supplied to the motor or motors exerting a forward motoring torque thereby minimizing the amount of power necessary to be supplied by the main generator. The power saving resulting from such an arrangement, as compared with one in which the braking effort is dissipated in heat, has been shown to be substantial over a course consisting of a large number of turns, such as may characterize the run of a military tank, or the like.

While controlled current systems of the general type shown in the British patent above referred to are suitable and satisfactory for vmarine applications, they have not heretofore been applied to land vehicles or to industrial applications in spite of their obvious advantages with respect to maneuverability. The reason is evident when it is appreciated that, while the system of the, British patent meets the principal demands of maneuverability, it is not characterized by rapid speed of response since such response is not ordinarily necessary in ship propulsion appatus. A land vehicle on the other hand, especially one which is to be operated in close proximity to a number of other vehicles and over irregular terrain, must possess not only maneuverability but also an extremely rapid rate of response. Furthermore, certain characteristic problems of land vehicle operation, such as overspeeding in running downhill, must be provided against by suitable protective means. Similar problems also are encountered in certain industrial processes where the load at times overhauls the motor.

Accordingly, it is a general object of our invention yto provide an improved electric power system of substantially unlimited flexibility which is reliable, easy to operate and possessed of a very rapid rate of response.

It is another object of our invention to provide a new and improved traction motor control system having a continuous transition from accelerating to electric braking operation and arranged for prompt and simple transfer from one phase oi operation to the other at any time without danger of overloading or overspeeding of any .ot the elements of the system.

It is still. anotherobjectof our invention to provide an' electric traction motor control system for a plurality of motors so arrangedv that any or all motors may be independently transferred between accelerating to electric braking operation substantially instantaneously and in a continuous manner without overloading or overspeeding of any part of the system.

It is a still further object 'of our invention to provide a gasor Diesel-electric power system arranged to permit maximum utilization of available engine horsepower over a wide range oi vehicle speed while permitting rapid and continuous transition from motoring to electric braking operation at any speed without overloading any part of the system.

It is another object of our invention to provide in a gasor Diesel-electric controlled current system means for providing a maximum current limit and means forchanging the current limit value in accordance with vehicle speed.

It is a still further object of our invention to provide, in a gas-electric or Diesel-electric power system for a self-propelled vehicle, new and improved means for limiting the vehicle speed.

Our invention also has for its object the provision, in a controlled current electric power system, of means for automatically establishing electric braking operation upon unloading of the motors and speeding up of the vehicle despite the fact that the operator maintains the controller in the full forward motoring position.

It is still another object of the inventon to provide a controlled current electric power system means for preventing destructive overspeeding of the prime mover by motoring of the generator during electric braking operation.

It is a stili further object of our invention to provide in a gas-electric or Diesel-electric power system for a self-propelled vehicle, new and improved means for increasing the torque speed ratio of the propelling motors.

According to our invention the above and other objects are attained by providing an electric power system, and particularly, a gasor Dieselelectric power system comprising a prime mover, a generator, and one or more traction motors in which a unidirectional direct current of continuously controlled and definitely limited magnitude is circulated through the motor armatures, and means are provided for separately controlling the excitation of the motor ileld windings independently to determine the speeds, torques, and directions of rotation of the motors. By proper selection of forward and reverse torques for the separate motors, steering and fordirect control of the continuously circulating armature current. To facilitate steering, means are also provided for automatically increasing the torque of the motors on one side of the vehicle whenever the torque of the motors on the other side of the vehicle is decreased.

Preferably we provide an engine driven generator having an output circuit including the armatures of all the traction motors connected in series circuit relation. The generator may be f the constant current type, althoughA for more eilicient utilization of the maximum available engine horsepower over a wide range of vehicle speed, it is preferable to limit the voltagecurrent characteristic by maximum available engine horsepower in the 'region between maxlmum generator voltage, es determined by saturation, and maximum generator current, as determined by armature reaction or a suitable current limit circuit. Each traction motor is, also aaeaeia provided with a separate engine driven exciter' provided with manually controlled excitation means to permit a smooth or continuous transition of motor excitation from maximum forward excitation to maximum reverse excitation. For

any predetermined setting of the manually adjustable motor field control means, the motor exciter field and hence the motor ileld energization may be constant or may be connected to vary in accordance with motor current, motor voltage, or motor speed, or any combination of these. In this system line current is limited by an electric valve spill-over circuit for reducing the generator excitation upon occurrence of a predetermined current, the spill-over value being reduced as vehicle speed increases. An electric valve spill-over circuit is also arranged to reduce the power output of 'the motors whenever the vehicle attains a predetermined limiting speed.

For a more complete understanding of our invention and a further appreciation of its objects and advantages, reference should now be had to the following detailed specification taken in conjunction with the accompanying drawings in which Fig. 1 is a schematic circuit diagram of connections for an electric power system embodying our invention; Fig. 2 is a simplified circuit diagram of the current limit circuit; F18. 3 is a simplified schematic circuit diagram of the vehicle speed limit circuit; Fig. 4 is a simplified schematic circuit diagram of the main motor tleld energizing circuit; and Figs. 5, 6, 'I and 8 are graphicall representations of various operating characteristics of the system.

Referring now to thedrawings, and particularly to Fig. 1, we have illustrated schematically therein a Dieselor other gas-electric power system for a self-propelled vehicle. The system 40 comprises a prime mover I0 of the internal combustion engine type, suchas a Diesel engine. r

arranged to drive through a common shaft Il a main generator I2, a generator exciter I3, and a plurality of traction motor exciters Il and I5. An output circuit from the armature of the generator I2 is completed through the armatures of a pair of traction motors I6 and I1. The output circuit of the generator I2 is a series or loop circuit and includes a commutating pole winding I8 for the generator I2 and commutating pole windings I9 and 20 for the motors I8 and I1, respectively. The generator output circuit may be permanently closed as shown, or if desired, suitable disconnecting switches may be included. We have also shown permanently connected in the output circuit a braking resistor 2l arranged to be shunted during motoring operation by the contact 22 of a braking contactor B. It will be understood, of course, that the representation of a pair of traction motors I6 and I1 is purely diagrammatic, and that each motor shown on the drawings may represent either a single motor or a group of motors arranged either in series or parallel circuit relation. For purposes of illustration it may be assumed that each motor or motor group I6, l1 is arranged to drive the track on one side of a. track-laying vehicle, such as a tractor, military tank, orv the like. For example, the motor I6 may drive the left-hand track, while the motor I1 drives the right-hand track.

It is well understood that an internal combustion engine of the type described, when its speed is maintained substantially constant by a'governor, is capable of delivering at its output substantially constant maximum horsepower, with gine I0, the setting of the governor being manually controllable by means of a foot pedal 26 through a'speed setting linkage 2l to select the desired constant speed. By way of example, we have shown a flyball governor arranged to rotate a governor shaft 28 through a lever 23. The governor is biased to a normal collapsed position by a tension spring 28. To the shaft 28 is rigidly connected an arm 30, and spring biased into following engagement with the arm 30, is a U- shaped link 30 which is loosely mounted upon the shaft 28 and rigidly connected to the throttle lever 23. This mechanism is so arranged that upon a decrease in engine speed the inward movement of the iiyballs will rotate the shaft 28 in a clockwise direction as shown on the drawings, thereby to open the throttle and bring the engine speed substantially back to normal. The link 30 is also provided with a projection 3I arranged to engage a fixed stop 32 in theiull open position of Athe throttle, thereby to prevent further movement of the throttle 23 but to permit overtravel of the lever 30 for a purpose to be more fully described hereinafter.-

The speed setting of the governor 25 is controlled by a tension spring 33 connected between the link 29 and a bell-crank lever 34 in aiding relation with the spring 29. The bell-crank lever 34 is connected through a suitable linkage, such as a hydraulic system 35, to the foot pedal 26 and is so arranged that depression of the foot pedal applies tension to the spring 33. We have also shown connected to the speed setting bell-crank lever 34 through a lost-motion linkage 36 a throttle'solenoid 31. As will be explained more fully hereinafter, the throttle sole noid 31 is arranged to apply to the speed setting spring 33 an initial tension as soon as excitation is applied to the field winding of the generator I2, thereby to raise the engine speed to a certain predetermined minimum above idling speed. The lost motion of the link 36 permits further application of tension to the speed setting spring 33 through the foot pedal 26.

Generator field excitation The main engine driven generator I2 is pro- 38 which is in turn controlled by the energizationof a plurality of control field windings 39. 40

and 4I on the exciter I3. y

Preferably the exciter I3 is'a direct current generator of the type described and claimed in Patent 2,227,992 issued to Ernst F. W. Alexanderson and Martin A. Edwards on'January 7, 1941. Such a generator is of the armature reaction excited type and is provided with a pair of current supply brushes 42, a series load compensating winding 42a, and a pair of short circuited brushes 43. 'I'he control field windings 38, 4II and 1 4I are arranged to generate a voltage between the short circuited brushes 43, and the armature reaction ofthe current flowing through the short circuited connection as a result of this voltage sets up a flux in such a direction as to provide an output voltage at the load brushes 42 which is proportional to the excitation of the machine along its short circuit axis. One of the desirable characteristics of an armature reaction excited i generator of this type is that its output voltage 'responds very rapidly and with a high degree of amplification to any change in the energization of its control` field windings. For this reason machines of this type are frequently referred to as dynamo-electric amplifiers.

Referring now more particularly to the control field windings of the generator exciter I3, the

. winding 39 is a stabilizing or antihunt winding connected directly across the load terminals of the exciter in series with a capacitor 44. 'I'he -eld winding 39 is thus excited only upon a change in exciter voltage and in a direction to oppose the change. The main control of the generator exciter voltage is attained through the generator exciter forward field 4I and the differential ield 40. Fundamentally, the forward control field winding 4I is .excited from a substantially constant voltage source of control power, such as a battery 45, while the differential field winding 40 opposes the winding 4I and is connected in shunt to the commutating pole windings I8, I9 and 20 in the generator output circuit for response in accordance with the main line current. winding 40 is modified by certain auxiliary circuits which will be considered in greater detail hereinafter. Specifically, the forward control field winding 4I is energized across a, governor controlled potentiometer 46, which is connected across the terminals of the battery 45 through a circuit which may be followed from the positive terminal of the battery through a disconnect switch 4l, a suitable protective fuse 48, a wire B+, a normally open interlock contact 49 on the braking contactor B, a resistor 50, the potentiometer 46, a resistor 5I, and a groundwire 52 which in operation is connected to the negative terminal of the battery 45 through ground and one or the other of a pair of manually operable dead man interlock contacts 53, 54 which will be more fully described hereinafter. The control field winding 4I itself is connected across the potentiometer 46 through a pedal controlled variable resistor or rheostat 55 and a fixed resistor 56.

The various parts of the governor controlled mechanism are shown with the flyballs completely collapsed so that the throttle linkage is in the full open position and the overtravel linkage connected to the arm 3'II on the governor shaft 28 is in the position of maximum overtravel. When the engine is at standstill. the springs 29 and 33 normally hold the throttle and overtravel linkages in the positions shown. When the engine is set in operation the governor balls immediately fly out and close the throttle down to its idling position as determined by the initial tension of the springs 29' and 33. 1n moving the throttle toits idling position the governor also moves the slider of the potentiometer 46 to its extreme counterclockwise position as viewed at Fig. 1. It will now be observed that the potentiometer 46 ls arranged to effect no change inthe voltage applied to the control eld winding 4I 4during .movement of the throttle from its idling position to its full open position. If, however, whenthe throttle is in its full open The excitation of the differential fieldy position, the engine is still unable to maintain the desired speed, the governor 25, through the overtravel linkage 24, will continue to move the lever 30 and the slider of the potentiometer 46, even though no further movement of the link 30' and the throttle lever 23 is possible because of the stop 32. Such overtravel of the governor will, through the potentiometer 46, reduce the voltage applied to the generator exciter forward ileld winding 4I, thereby to reduce the excitation of the generator exciter I3, andthe main generator i2, and to permit the engine to again come up to the desired speed. By this arrangement the power demand of the engine driven generator I2 is reduced in the middle range of its volt-ampere characteristic where it normally exceeds the maximum available horsepower output of the engine I0, thereby to prevent slight engine stalling within this range and to permit maximum utilization of all available engine horsepower over a wide range of vehicle speed.

The-purpose of the rheostat 55 is to select a suitable generator excitation in accordance with the selected engine speed to permit easy starting of the vehicle and easy maneuvering at low speed. The rheostat 55 is arranged to reduce the generator exciter iield resistance over an initial portion of the speed range but to effect no change in the field resistance over the major .portion of the accelerator pedal travel. Thus as the slider of the resistor 55 is moved from its extreme counterclockwise idling position by accelerating movement of the' foot pedal 26, the resistance 55 is gradually -excluded from the circuit and the exciter excitation built up as the engine speed increases. V

Preferably, the resistor 55 is arranged to be completely cut out at a relatively low engine speed. For example, if the engine is arranged to have a minimum running speed of 1200 R. P. M. with the throttle solenoid 31 closed,

it has been found suitable to completely cut outV the resistor 55 at a selected low engine speed setting of approximately 1400 R. P. M., as has been indicated onthe drawings. ,Thus it will be observed that for pedal position beyond that corresponding to this selected low speed but less than full open neither the governor potentiometer 46 nor the rheostat 55 exerts any control of the energization of the control eld winding 4I so that the energization of the winding 4I is substantially constant within this range of throttle positions. It will be understood that variation of the eld resistance 55 is not fundamental but that, if desired, a fixed resistor may be substituted therefor. Beyond the full open position of the throttle the governor 25 functions to reduce the power demand of the generator to equal the maximum available horsepower output of the engine by control of the generator forward iield 4I through the potentiometer 46.

As previously mentioned, the control field winding 40 is a differential winding and is arranged when energized to excite the generator exciter field in opposition to the ilux of the winding 4I. The eld winding 40 is connected across the commutating pole windings I8, I9, and 2n in the generator output circuitl and is thus energized in accordance with main line current, thereby to cause the Aoutput voltage of the generator I2 t0 drop from a predetermined maximum value to a value just suilicient to supply the resistance drop of the output circuit when maximum current is flowing. The normal energization circuit of the winding 40 may be followed from the positive vterminal of the generator I2 resistor 51 to ground.

The speed responsive governing apparatus described above is claimed in a copending application of Martin A. Edwards, Serial No. 505,437, illed October 8, 1943, and assigned to the same assignee as the instant application.

Line current limit For the purpose of limiting the current in the generator output circuit to a. predetermined maximum value, supplementary means are provided for abruptly increasing the energization of the exciter differential winding 40 when the line current attains a-predeterminedmaximum value. The current limit circuit may be more readily understood from the simplified diagram of Fig. 2. At Fig. Z'the connection of the exciter differential winding 40 acros's the commutating pole windings I8, I9, and 20 in series with the resistor 51 is clearly shown. 4'To provide maximum current linut, the resistor 51 is shunted by a circuit including the battery 45, a rectifier bridge 58, and a normally open interlock contact 58 of the braking contacter B. As pointed out .hereinbefore, the braking contactor B is picked up during motoring operation so that the shunt circuit around the resistor 51 will be closed. It will be observed that both arms of the rectifier bridge are connected to block any flow of current from the battery 45 through the resistor 51. Furthermore, the generator polarity is such that when current is owing through the output circuit in the normal direction, the side of the resistor 51 connected to the negative side of the battery is negative, and the side of the resistor connected through the rectiiier bridge to the positive side of the battery is positive, so that the voltage drop across the resistor 51 bucks the voltage of the battery through the blocking rectier bridge 58, the rectifiers preventing any flow of current through .the loop circuit from the battery. It will now be evident that when the current flowing in the output circuit of the generator I2, and hence the current flowing through the exciter differential winding 40 and the resistor 51, attains such a value that the voltage drop across the resistor is greater than the voltage of the battery 45 by an amount suilicient to break down the rectiiiers in the arms of the bridge 58, current. will spill over from the positive terminal of the resistor 51 through the battery 45 thereby to provide a current path parallel to the resistor 51 and in series with the differential control field 40. By thus introducing a shunt connection around the resistor 51 the net resistance of the circuit through the control field winding 40 is reduced and the energization of the winding 40 is sharply increased thereby to diminish the excitation of the generator exciter I3 and cut down the excitation of the main generator I2. The main line current at which the above spill-over operation takes place is the maximum current limits of good commutationby the traction motors i6 and I1. It is well understood that the maximum current which may be satisfactorily.

commutated at high speed is less than that which may be satisfactorily commutated by the same machine at a lower speed. For this purpose we provide a tachometer generator 60 having a separately excited eld winding 6I energized directly from the battery 45 through the wire B+. Thetachometer generator 60 is driven from the shaft of one of the traction motors, for example, the traction motor I6, and thus supplies an output voltage proportional to vehicle speed for modifying the operation of the current limit circuit. From the discussion of the preceding paragraph it will be recalled that the shuntl circuit across the resistor 51 is connected through diagonally opposite points of the rectiiler bridge 58. These points have been identiiied as 62 and 68 in Figs. l and 2. Referring now particularly to Fig. 2, it will be observed that a potentiometer 64 having variable voltage tap 65 is connected directly across the output terminals of the tachometer generator 60 to supply to the other pair of diagonally opposite points 66 and 51 of the rectifier bridge 58 a suitable voltage proportional to vehicle speed. Due to the rectifying action of the bridge 58 this speed voltage.

may be regarded as appearing permanently between the points 63 and 62 of the bridge regardless of the direction of motion of the vehicle, the

point 63 being positive with respect to the pointv 62. From Fig. 2 it may also bel observed that in the loop circuit comprisingv the battery 45, the rectiiler bridge 58, and the resistor 51, the voltlage drop across the resistor 51 and the speed-voltage across the points 62 and 63 of the rectifier bridge 58 are connected additively and in opposing relation to the voltage of the battery 45.

lThus for current limit operation the total voltage available for bucking the battery voltage to effect a spill-over of current through the rectifier bridge and the battery from the positiveA terminal of the resistor 51 is increased in proportion to the speed of the vehicle by the voltagebetween the points 62 and 63 of the rectier bridge. Since the battery voltage is constant, it will be clear that as the vehicle speed increases, spill-over through the battery and hence current limit operation will take place with progressively smaller voltage drops across the resistor 51, that is, at reducedI main line current, since the voltage drop across the resistor 51 is proportional to the main line current. In this way the maximum current limit is reduced as the vehicle speed in creases.

LTraction motor field control Referring now to the field control for the traction motors I5 and i1, it may first be noted that each motor is provided with a main separately excited ileld winding, the motor I6 havingA a winding connected to theoutput terminals of the dynamoelectric exciter I4 and the motor I1 having a eld winding 1I connected to the output aseaeia cordance with the voltage drop across the cornmutating pole windings I8, I8 and 20 of the main generator output circuit. The energizing circuit for the windings 12 and 13 may be traced from the positive terminal of the engine driven` generator I2 through a resistor 16, a wire 11, and in parallel through a pair of variable resistors 18, 19 to the points 80 and 8i on the von the potentiometers 14 and 15 by means of manually positionable steering handles 83 and 84,

respectively. A

It is desirable at'this point to note certain mechanical features of'the steering handles 83 Y and 84.r Primarily these handles have for their purpose the determination of the standard of energization of the motor ileld windings 10 and 1I by selection from the potentiometers 1 4 and 15 of a suitable voltage proportional to line current for application to the motor exciter field windings 12 and 13. It will also be noted that the dead man" interlock switches 53 and 54 previously mentioned are actuated by handle levers attached to the steering handles and that these switches are arranged to be closed as soon f as the handles are grasped by an operator. Preferably, as indicated in the drawings, the steering handles 88 and 84 are biased to a normal position such that substantially full forward voltage is applied to the control field windings" and.

13. The steering handles are also arranged to operate a number of auxiliary switches and rheostats. For example, the rheostats 18 and 19 are arranged to be fully in circuit when the steering handles 83 and 84 are in their normal posi tions and are connected respectively to the handles 83 and 84 for movement therewith to cut out resistance as the handles are drawn back. toward the null points of the potentiometer bridges. The rheostats 18 and 19 are arranged to be fully cut out when the steering handles reach the null points. The purpose of these rheostats will be more fully described hereinafter in connection with automatic steering operation. In addition to the rheostats 18 and 19, the steer ing handles 83 and 8.4 are also arranged to actuate potentiometers 85 and 86, respectively, and switches 81 and 88, respectively.y The potentiometers 85, 86 and the switches 81, 88 are in circuits which will be described hereinafter', but

it will be suflicient for the present to note that.

the sliders of the potentiometers 85 and 88 are normally at their maximum voltage positions and, since the forward halves of these potentiometers are dead," the maximum voltage position is maintained so long as the steering handles are forward of their null points. The potentiometers 85 and 86 become effective to reduce the tapped voltages as the steering handles are moved beyond their null points and--into their reverse quadrants. The switches 81 and 88 are single-pole, double-throw switches which are normally closed on one pole so long as the steering handles are in their normal forward positions and are thrown tothe other poleas soon as the vconnected steering handle is moved in the reverse direction from the normal position.

lt'will be clear from the above description of the potentiometers 14 and 15 that with the steering handles 83 and 84 set in any predetermined position, the energization of the main control field windings 12 and`13 of the motor exciters i4 and l5 will vary in accordance with the mag` 'nitude of the main line current. Furthermore,

since the exciters i4 and l5 serve as dynamoelectric amplifiers, it will be clear that the energization of the motor iieid windings 10 and 1| is proportional to the armature current of the motors, and that, except as modified under certain conditions explained hereinafter, the exci- 'tation characteristic of the traction motors |6 and is similar to that of a series motor. It wlil be understood that the steering handles 89 and 84 are independently controllable so that the standard of excitation of each motor may be selected independently and set at any point between full forward field and full reverse field. The energization circuit for .the motor exciter windings 12 and 13 is shown in simplified form at Fig. 4. While Fig. 4 shows only a single potentiometer bridge, it will be understood that the potentiometers 14 and 15 are connected in parallel circuit relation.

The series excitation characteristic of the traction motors i6 and |1 is desirable because it increases the speed up to which maximum utilization of available engine horsepower may be obtained. However, it has been found that even higher motor speeds than may be thus obtained are desirable at the point of maximum utilization of engine power. This demand arises in part from the fact that saturation of the motor y fields at low vehicle speed in an eil'ort to obtain maximum tractive effort forstarting impairs to some' extent the proportionality between line current and the motor flux. AccordinglyJ in order further to increase the motor speed up to which maximum utilization of engine horsepower may be obtained, we provide additional means for exaggerating the series excitation characteristic of the motors by controlling the motor excitation in inverse proportion to vehicle speed as well as in direct proportion to the main line current. For this purpose we utilize the voltage of the tachometer generator S0 to energize, in proportion to vehicle speed, a pair of differential control iield windings 90 and 9| on the motor exciter-s i4 and i5, respectively. Referring to Figs. l and 3, it will he observed that the control eld windings B and 9| are connected in series circuit relation across intermediate points 92 and 93 of a bridge circuit the line terminals 94 and 95 of which are connected to the terminals of the tachometer generator 60. Fig. 3 clearly shows the bridge circuit which comprises a pair of resistors 96 and 91 connected in series between the' terminals 94 and 95 and a resistor 98 connected in series with a rectifier 99, also between the ter- `minals 94 and 95. The rectiner 99 is connected normally to conduct, the terminal 94 being positive. The arms of the bridge are so proportioned that the point 92 is normally positive with respect to the point 93 and the motor exciter neld windings 90, 9| are connected between these points. The energizing circuit for the windings 90 and 9| may be traced from the bridge terminal 92 through a rectifier |00, the control field windings 90 and 9| in series, -the steering handle switch 88 in its'normal'position and the slider of the steering handle potentiometer 85 to the both potentiometers through the switches 81 and* 88, depending upon the positions of the steering handles 83- and 84. The purpose of the potentlometers 85 and 88 will be later explained. but for the present it may be assumed that the potentiometers are ineiiective and that the steering handles are in the full forward positions shown in the drawings. Thus, as the vehicle speed and tachometer voltage rise, the voltage across the bridge points 92, 99 increases proportionately. As indicated at Fig. 1 the diiierential control field windings 90 and 9| are energized in opposition to the main field windings 12 and 13, respectively, so that, as the energization of the windings 90 and 9| increases with increasing vehicle speed, the net excitation of the traction' motor exciters I4 and i5, and hence the excitation of the traction motors IS and |1 then selves, is reduced in proportion to the speed of the vehicle. It will be borne in mind that this effect is superposed upon the reduction of trac tion motor excitation due to thedecrease in main line current as the vehicle speed increases and'produces an exaggerated series motor characteristic.

The reason for making use of the rectier 99 as one arm of the bridge circuit of Fig. 3 will become evident from an examination of the curves o! Figs. 6 and 1. The rectifier 99 serves as a nonlinear resistor which decreases its resistance as the voltage across the bridge increases. Referring first to Fig. 6, we have here shown the relation between vehicle speed and line current from zero speed upto maximum vehicle speed. The maximum vehicle speed is indicated at Fig. 6 by a line |0|. The manner in which the vehicle speed islimited to this maxirnurn will be described hereinafter, but for thc present it is suicient to note that the broken line curve |02 of Fig. 6 indicates the manner in which the line current would diminish as the vehicle speed increases were it not for the additional iield weakening due to the energization of the motor exciter field windings and 9|. Also at Fig. 6 the straight 'line |03 indicates the maximum current limitwhich, as has been heretoore explained, diminishes as the vehicle speed increases. graph, the excitation of the control field windings 90 and 9| increases with vehicle speed and this increase is substantially linear over a portion |04 of the curve of Fig. 7, so that the line current of Fig. 6 follows a curve |05 rather than the curve |02. As indicated at Fig. 6, the departure of the curve |05 from the curve |02 becomes more pronounced as the energization of the field windings 90 and 9|, as shown in Fig. 7. increases. Thus, due to the effect of the field windings 90 and 9|, the line current begins to show an increase at a portion |05a of the Curve |05, and begins to approach the current limit value. Since it is undesirable that the current limit be exceeded at this point, further increase in the energization of the eld windings 90 and 49| is prevented and, in fact, a net decrease in As described in the preceding para- 92 with respect to terminal 93. Indeed. the effect of the rectier 99 may be so pronounced as to actually reduce the voltage across the control filed windings 90 and 9| despite further increases of tachometer voltage (see the portion I04a ci the curve |04, Fig. 7). As indicated at Fig. 6, then, this hasthe eiiect of again reducing the line current at a portion |05b of the curve |05. By thus limiting, and in fact reducing, the energization of the windings 90 and 9| in the high speed range, an increase of the line current to the limiting value is avoided.

The non-linear bridge circuit motor iield control system described above is claimed in a copending application of George M. AdamsI Serial No. 505,469, iiled October 8, 1943, andv assigned to the same assignee as the instant application.

, Vehicle speed limit In addition Lto their function of exaggerating the series excitation characteristic of the traction motors, the motor exciter control field windings 90 and r9| are connected in circuit with'the control battery 45 to provide a vehicle speed limit control and indication. For this purpose the voltage of the tachometer generator 60 is matched against the voltage of the battery 45 through'a loop or spill-over circuit including the control field windings 90 and 9| of the motor exciters I4 and I5 and a pair of blocking rectiers Illi and Ia (see Figs. 1 and 3), the rectifier I05a being in the rectier bridge 58. The blocking rectifiers normally prevent a flow of current from th'e battery through the tachometer generator 00 and the ileld windings 90 and 9| but permit a spill-over current from the tachometer generator through the battery and the control eld wind ings 90 and 9| to supply supplementary excitation to the field windings whenever the vehicle speed is such that the tach'ometer voltage, or a suitable portion thereof, exceeds the battery voltage. The spill-over connection may be traced at Figs. 1 and 3 from the positive terminal of the tachomete'r generator 60 through a wire |01, the rectiiler |06a, the wire B+ and the battery 45 to ground, and from ground through the rectifier I 06, a suitable visual or audible speed indicating device'I08, the motor exciter control field windings 90 and 9|, the steering handle switch 88, the variable steering handle resistor 85, and the resistor 91 to the negative terminal of the tachometer generator 60. It will now be clear that the rectifier |00 is included in the normal or bridge excitation circuit of the iield windings 90 and 9| for the purpose of preventing short circuiting of these iield generator voltage increases or decreases proportionately.

The operation of the speed limit circuit may now be followed. It will be recalled that independently of any energization of the motor exciter control iield windingsl 90 and 9| through the speed limit spill-over circuit traced in the preceding paragraph, these eld windings are continuously excited through the bridge circuit of Fig. 3 in the manner indicated at Fig. 7. Re-

ferring now to Fig. 6, as the vehicle speed limit is ,approached the line current is in the region |b of the curve |05. If now th'e vehicle speed f becomes such that the tachometer voltage causes windings through the rectifier 99 upon spill-over in speed limit operation. From the above circuit it will be evident that the rectiiiers |00 and I 06a are so arranged that current cannot flow from the battery 45 through the tach'ometer generator 00. On the other hand, when the vehicle attains such a speed that the voltage at the output terminais of th'e tachometer generator attains a value greater than that of the battery voltage, these two blocking rectiers will permit current' to spill over from the tachometer generator through the battery, the overspeed indicating device |09, and the motor exciter control iield windings 90 and 9|. Furthermore, it is to be noted that the `veh'icle speed at which such spill-over will occur is substantially unaffected by changes in the battery voltage because of the fact that the tachometer generator is battery excited, so that as the battery y voltage increases or decreases, the tachometer current to spill4 over through the speed limit circuit thereby to energize the overspeed indicating device |08 and to superpose additional energiza tion upon the normal energization of the motor excited eld windings 90 and 9|, the total energization of the windings 90 and 9| will increase sharply as indicated at the portion I04b of the curve of Fig. 7. Such increase in the energization of the differential .windings 90 and 9|V will cause a sharp decrease in the net motor exciter excitation, and hence in the net motor excitation, so that the line current will increase sharply as indicated at I05c of Fig. 6. This sharp increase in the line current will bring the line current to the limiting value at the speed of operation, as indicated by the curve |03 of Fig. 6, so that the current limit circuit previously described will be brought into operation. Thus with the motor current limited at the current limit value, the motor power is determined by the motor voltage. The motor voltage is proportional to the motor speed and to the excitation of the motor field windings, so that with the motor speed remaining at the maximum value andthe motor excitation severely limited by the increased excitation of the motor. exciter fields 90 and 9|, the motor power will drop oii, thereby to reduce the vehicle speed. When the vehicle speed is thus reduced, the tachometer voltage is reduced below th'e spillover value and supplementary excitation of the motor exciter fields 90 and 9| ceases. It willthus be clear that the system will regulate on this point to maintain the vehicle speed at its maximum value.

The above operation of the system under speed limit conditions will be clarified by a considera tion of Fig. 5. This ligure is a more or lesss conventional diagrammatic representation of the over the widest possible range' of vehicle speed.

engine is capable of supplying and that the only way that the curves may be caused to coincide, as they must, is for the power output of the gen- `erator to be slightly diminished by slight engine stalling within this range. Since such reduction in the engine speed also -cuts down the available power output from the engine, it is desirable if possible to cut down the generator demand so that it just equals thel engine output. In the system disclosed provision is made for reducing the generator excitation by means of the governor operated potentiometer 4B previously described. By way of example, the curves A and B of Fig. 5 have been drawn for the full open position of the engine throttle. With the throttle in this position the potentiometer 46'has effected no change in' the potential applied to the generator exciter field di. However, aspreviously explained, the governor is provided with a certain dg'ree of overtravel so that ifl with the throttle in the full open position, the engine speed still cannot be maintained, the lever 29 attached to the governor moves the slider of the potentiometer 46 to effect 'a reduction in the energization of the generator exciter field winding 4I without further opening of the engine throttle. By thus reducing the ex citation of the main generator I2, the power demand oi the generator is reduced to a point where it just equals the available power output of the engine. This action is of a regulatory nature and results in bringing the generator volt-ampere characteristic into coincidence with the available power output of the engine over the constant power range C of the resultant generator volta'mpere characteristic shown in solid lines at Fig. 5'. It will be lunderstood that in the region D the voltage of the generator is limited to a predetermined maximum value by saturation of the generator field poles. The portion E on the curve of Fig. 5 is determined by operation of the current limit circuit at zero or low vehicle speed. It will be understood that as the vehicie speed increases, the current limit is reduced as indicated by tho constant lines E1 and E: of Fig. 5. The portion F oi' the generator volt-ampere characteristic represents the pure resistance drop through the armatures of the traction motors along which the iine current builds` up in proportion to generator voltage when the vehicle starts from standstill.

'Referring again to the operation of the vehicle speed limit with 'the curves of Fig. 5 in mind, it will he evident that, ii' the vehicle is operating at a point i iii on the generator volt-ampere characteristic when the vehicle speed reaches its maximum value, the sharp increase in motor exciter differential field excitation produced by the speed limit circuit and indicated at' Fig. 7, and the consequent sharp increase in line current indicated at Fig. 5, will cause the operating point I I of Fig. to move down the curve C to the maximum speed current limit curve E2, and hence down the current limit curve En to a point such as III oi 5. It will he clear that the system power represented at the point III is less than that represented by the constant power curve C so that the power output of the motorslwill be reduced and the vehicle will slow down as previously described.

Stabilization o! motor excitation lt will be understood from the above that when the vehicle is moving forward under power, the counterelectromotive force of the motors is almost equal and opposite to the output voltage of the engine driven generator I2. Since the motor voltage is proportional to the product of the motor flux and motor speed, it is apparent that when the vehicle is traveling at a relatively high speed the motor fiux is quite weak while the generator I2 is operating under partial saturation in its upper range of voltage. Under such conditions it will be evident that the motor exciters Il and i! are capable of forcing an extremely rapid change of motor field flux withl only a relatively small change in motor field energization; while because of the generator ileld saturation, a relatively large voltage change at the generator exciter Il is necessary to produce a proportional change in the generator field flux. Accordingly, if the excitation of the traction motors is sud- 'denly reduced or reversed to initiate dynamic braking operation, it will be'clear that a transient current of considerable magnitude might flow in the generator output circuit because of the fact that the motor field flux will reverse much more quickly than the generator flux can be reduced by operation of the current limit circuit. For the purpose of bringing this undesirable condition within permissible limits, the motor exciters Il and I 5 are provided with stabilizing control field windings IISand I I6, respectively. Referring to the drawings it will be observed that the positive terminal of the motor exciter I5 and the negative terminal of the motor exciter I4 are connected together and to ground through a wire II'I, while the positive terminal of the motor exciter Il is connected through the stabilizing control field winding IIS, a condenser II.8, andthe stabilizing. control field winding I I6 to the negative terminal of the motor exciter I5. Thus when the system is operating under steady-state conditions, no current flows through the stabilizing windings IIS and IIS, the condenser I IB merely being charged to a potential equal to the sum of the'potentials across the output terminals of the motor exciters Il and i5. The stabilizing circuit is a loop circuit comprising the armatures of the motor exciters Il and I5, the control field windings i I5 and IIB, and the condenser III, all in series circuit relation, the

Vcircuit being tied to ground between the armatures of the motor exciters by the wire III.

It may now be observed that if the voltages across the output terminals oi' both motor exciters are changing in the same sense, that is, if increased or decreased motoring or braking torque is simultaneously being called i'or on both exciters, or if the excitation of both exciters is suddenly and simultaneously changed from forwardl excitation to reverse excitation to shift from motoring to braking operation, the condenser IIS is charged or discharged (depending upon the direction of the change) through the stabilizing control field windings II5 and IIB. These stabilizing windings are so arranged that the com- DOnents of field excitation which they produce under such conditions of change in the motor exciters I4 and I5 tend to oppose the change 0f voltage in each machine. In this manner the rate of change of output voltage of the motor exciters is sufiiciently decreased so that the generator exciter differential eld M is able to make the ilux of the generator field 3B follow the motor flux sufllciently closely to prevent an exceedingly high transient current during a rapid change of motor excitation. If, on the other hand, the voltage of only one of the motor exciters is changed, either by increasing or decreasying the field excitation of that motor, while the voltage of the other motor exciter is maintained constant, the condenser i I 8 will also undergo a change in charge. In this case however the current through the stabilizing field windings IIB and IIB will be in the same direction in both windings, so that the current through the stabilizing winding of that motor exciter whose voltage is undergoing change will tend to oppose the change, while the current through the stabilizing winding of that motor exciter whose field exci-` tation is not otherwise undergoing change will l tend to produce a change in voltage of that exciter in a direction opposite tothe voltage change in the iii-st exciter. This latter effect is desirable in connection with steering operation, as will appear hereinafter. Steering is effected by moving one steering handle to change the torque of the associated motor. In order that maximum steering eiliciency may be effected, means are preferably provided for automatically changing the torque of the other motor in the opposite direction without necessitating movement of the other steering handle. It will be clear that vthe .above transient effect aids such a change.

While in its broader aspects the stabilizing arrangement described above is our invention and is claimed herein, the speciiic embodiment described is the invention of George M. Adams and is claimedin the aforementioned application Serial No. 505,469.

Steering As briefly mentioned in the foregoing paragraph, steering of the vehicle is effected by actuating one or the other of the steering handles to reduce or reverse the torque of the associated motor without reducing the torque of the other motor, thereby causing the vehicle to turn toward A understood that if the vehicle is traveling in the reverse direction, steering is effected by moving one handle towardor into the forward quadrant.

In order to eiiect steering with a minimum of disturbance of the main line current, it is desirable to maintain constant the net motor voltage during steering operations. Means are thereforey provided for automatically increasing the excitation and hence the torque of the motor whose steering handle is not disturbed when the voltage and torque of the other motor is decreased. For example. if the steering handle 84 is drawn toward the null point of the potentiometer 15 while the steering handle 83 is left in the position shown, the resistance of the rheostat 18 will be gradually diminished and will be completely cut out when the handle 84 reaches the null point. Referring now to Fig. 4, it will be evident that as the rheostat 18 is cut out, thev voltage applied to both doubley potentiometers 14 and 15 is graduallyincreased. Thus, since the steering handle 83 is not moved, it will be clear that the ileld excitation of the motor exciter I4 is increased, thereby to increase the voltage and torque of the motor I8. Of course, the-voltage applied to the potentiometer 14 is also increased, but the reverse movement of the handle 84 takes vprecedence so that the resultant excitation 'of the motor exciter I5 is decreased, thereby decrease ing the voltage and torque of the motor i1. By suitable proportioning oi' the various elements of the system, the rheostat 19 may be caused to increase the voltage of the motor I8 by the same amount that the voltage of the motor I1 is decreased by the steering handle 84 until theinullpoint is reached and the rheostat 18 is completely cut out. This condition is illustrated at'Fig. 8 which shows the relation between steering handle travel and motor voltage for the condition assumed. 'It will be clear that if the handle 83 is moved and the handle 84 left stationary, the rheostat 18 will have the effect of increasing the voltage of the motor I1.

The rheostats 18 and 19 are not effective when either steering handle is in the reverse' quadrant, since these resistors are completely cut out when the handles reach the null point. However, means are provided for maintaining the net motor voltage substantially constant even if one steering handle is drawn back into the reverse quadrant to eiect vbraking operation of one motor for very abrupt steering. For this purpose the potentiometers 85 and 86, which are inefi'ective so long as the steering handles are in the forward quadrant. are brought into operation. These potentiometers, which are connected in parallel with the rcsistor88, have the effect of reducing the excitation of the motor exciter diierentiai control eld windings and 9i whenever one of the steering handles is moved into the reverse quadrant while the other is left in its normal position. Continuing with the example as sumed, let it be supposed that the steering handle 84 is moved into the reverse quadrant. Referr-ing now to Fig. 3, it will be evident from the foregoing description of the apparatus connected 'to the steering handle 84 that the selector switch sa will have beenthrown into engagement with the contact I 2i as soon as the steering handle 84 was moved `backward from its normal position,P

thereby to connect themotor exciter elds 98 and 9i to the bridge terminal 83 through the slider of the potentiometer 86 rather than the slider of the potentiometer 85. This changed connection has no effect until the steering handle reaches its null point, since the potentiometer 86 is effec-y tive only in the reverse quadrant. However, as soon as the steering handle is moved into the reverse quadrant, the slider of the potentiometer 88 moves over the live section of the potentiometer from the voltage of the point 93 toward the higher .voltage of the bridge terminal 84, thereby to diminish the voltage across which the field windings 90 and 8l are connected. Since these iield windings are differential windings, such decrease in their energization produces a net increase in the excitation of both motor exciters. In this way the voltgage of the motor I8 continues to increase, while, as previouslydescribed in connection with the forward quadrant movement, the resultant voltage of the motor I1 increases in the reverse direction as the steering handle 84 approaches its full reverse position.. As indicated at Fig. 8, the voltage of the motor i8 increases somewhat nonlinearly because of motor saturation after the steering handle 84 passes thenull point.

ItA will be understood that if instead of moving the steering handle 84, the operator has moved the steering handle 83 into the reverse quadrant and left the steerlng handle 84 stationary the potentiometer v85 would have produced the same effect as Just described in connection with the po-y tentiometer 86. Furthermore, if both steering handles were simultaneously moved into their reverse quadrants to effect braking operation without steering, both selector switches 81 and 88 would be actuated, that is, the switch 88would engage the contact IZI and the switch 81 would engage its contact |20, thereby to disconnect the Dynamic braking operation Dynamic braking operation without steering is effected by drawing both steering handles 83 and 8d simultaneously from their forward quadrants into their reverse quadrants. During this operation the potentiometers 85 and 86 are ineilective to reduce the excitation of the motor exciter fields 90 and 5| .because they are bypassed by the switches 81 and 88, as explained in the preceding paragraph. However, the rheostats 18 and 19 are effective, so long as the steering handles are f still in their forward quadrants, to increase the voltage applied to the motor field windings 10 and l I, as explained heretofore in connection with the steering operation. By thus increasing the standard of excitation of the motor fields, a high braking torque is maintained at relatively low line currents.

While the braking torque at low line currents is maintained in the manner described above, means are also provided for limiting the braking torque at high line currents. The need for such a limitation will be appreciated when it is realized that under bralnng conditions the field excitation of the traction motors I6 and ll is reversed so that these motors act as generators, their voltages adding to the voltage of the main engine driven generator i2. Under these conditions the series excitation characteristic of the traction motors becomes a series generator characteristic having a marked cumulative effect; that is, as the excitation of the traction motors is increased in a braking sense, the line current is increased, and as the line current increases, the excitation or" the traction motors is further increased. This effect is aggravated by the transient overshooting of the line current limit previously explained as resulting from the inability of the generator to reduce its voltage as quickly as the motor voltage is reduced when making a sudden transfer to dynamic braking operation. While the motor exciter stabilizing windings i I5 and IIS limit this overshooting to permissible values, it is also desirable to limit the braking torque when such high line currents exist. Indeed, it has been found desirable to limit the braking torque at high. line current even though the line current does not exceed the current limit value. For this purpose we provide means for limiting the excitation of the traction motors i6 and I1 to a predetermined maximum value, so that the series excitationcharacteristic of the traction motors is transferred to a constant or shunt excitation characteristic upon the occurrence of a predetermined high line current. Fig. 4 shows the manner in which the energization of the main motor exciter control field windings 12 and 'i3 is limited to a definite maximum value. As has been previously pointed out, Fig. 4 shows the potentiometer bridge 14 (the potentiometer bridge 15 being connected in parallel circuit re lation) connected across the interpole windings I8, I9 and 20 in the generator output circuit in series with the resistor 16 and the rheostats 18 and 18. Furthermore, it will be now understood that normally the basic series characteristic of the motor exciter winding 12 arises from the fact that the potential at the point Il of the potentiometer 14 is proporional to the voltage drop across the commutating pole windings Il, I9 and 20 as derived through the resistors 1l, 1l and 19. As explained heretofore, this proportionality is modified to some extent in braking operation by reduction of the resistance of the rheostats 1l and 19. However, in order to limit the potential at the point l0 to a predetermined maximum value, this point is tied through a blocking rectifier |26 to a point of intermediate voltage |21 on the battery 45. From Fig. 1 it will V be evident that the point |21 has a fixed potential slightly above ground potential as determined by the battery 45. So long as the potential of the point remains below that of the point |21 on the battery 46, current cannot flow from the battery through either the potentiometer or the commutating pole windings I8, i9, and 20 because of the presence of the blocking rectifier |28. However. should the line current through the commutating pole windings i8, I9, and 20 attain such a value that the potentlal at the point B0 tends to attain a value in excess of the Ipotent-ial at the point |21, current will spill over from the point 80 through the rectifier |26 and the connected portion of the battery 45 to ground, thereby limiting the potential of the point 80 to a predetermined maximum value. After this operation the potential of the point 8D remains substantially ilxed regardless of the value to which the line current. and hence the potential drop across the commu'- tating pole windings I8, I9 and 20, may go. Therefore when the line current attains a value sufficient to cause such spill-over, the energization of the motor exciter control field windings 12 and 13, and accordingly the excitation of the traction motors I6 and Il, attains a fixed maximum value and is possessed of a shunt characteristic. In this manner the undesirable cumulative effect of motor field strength are mlnimized when going into dynamic braking operation. It will be understood that the excessive potential at the point 80 may not be due alone to a simple resistance drop across the commutating pole windings I8. IQ and 20, but that upon a very rapid change in line current, as due to a quick reversal of the traction motor field windings, such an excessive potential may arise in part from the inductive drop across the oommutating pole windings. The spill-over circuit through the rectifier |28 also guards against such excessive inductive transient potentials.

Under conditions of rapidly changing line current in suddenly going into braking operation as described above,v the inductive potential appearing across the interpole field windings I8, Il, and 20 isfutilized to hasten the decrease in excitation of the generator I2 to its steady-state value. For this purpose the energization of the differential field winding 4I) of the generator exciter I3 is arranged to go to exceedingly high values, It will be recalled that the differential field winding 40 of the exciter I3 is energized across the commutating pole windings I8, I9 and 20 so that any inductive voltage drop appearing at the positive terminal of the generator I2 will provide a transient increase in energization of the winding 40 and hence a rapid decrease in the net excitation of the generator exciter I3 and the generator I2. By forcing an increased current through the differential field winding 40, the inductive voltage at the positive terminal of the generator I2 also appears across the resistor l1 in the generator exciter differential excitation Operation In view of the foregoing detailed description of the various parts of the system and their manner of connection and mode of operation,

, the operation of the system as a whole will be vided with deadman interiocks 53 and 54 normally biased to an open position.

Assuming first that the vehicle is at standstill and that the engine |0 is running at idling speed, motoring operation is initiated simply by grasping one or both of the steering handles to close one or both of the dead man interlocks 53, 54. The interlocks 53, 54 are connected in parallel circuit relation, and closure of either interlock completes an energizing circuit for the main control fieldwinding 4| of the generator exciter I3 through a circuit which has been previously traced. Simultaneously an energizing circuit is completedrfrom the positive terminal of the battery 45 through an actuating coil |30 of the throttle'solenoid 3l to the wire 52 and through the dead man interlocks 53 and 54 to ground. This circuit also includes a normally closed interlock Contact |3| on an overspeed device |32. The overspeed device |32 is arranged to be actuated upon overspeeding of the engine I0 to deenergize the throttle solenoid 31. As has been explained hereinbefore, actuation of the throttle solenoid 31 adjusts the setting of the governor 25 to maintain a predetermined minimum running speed of the engine |0, thereby to assure adequate power for steering or braking even though the accel erating pedal 26 should be accidentally released.

Closure of one of the dead man interlocks 53, 54 to initiate motoring operation also completes an energizing circuit through a reverse power relay RPR'for an actuating coil |35 `oi? the braking contactor B. The braking contacter B is normally open. but is closed during motoring operation to shunt the braking resistor 2| in the main generator output circuit. Thereverse power relay RPR is provided with a polarizing winding |36 and a directional winding |31. Upon closure of one of the dead man interlocks 53 or 54. an

- energizing circuit is completed for the polarizing winding |36 from the wire'B+ through a normally closed interlock contact |38 on the braking contactor B and the winding |36 to the wire 52 and hence t0 ground. When so energized, the

reverse power relay RPR picks up and is retained in its actuated position by'a holding current of limited value through a resistor |39 which shunts the normally closed interlock contact |38. Uponactuation of the relay RPR an energizing 'circuit is completed for the actuating winding |35 of the braking contacter B through a circuit which may be traced from the wire B+ through a normally open interlock contact |40 on fthe reverse to complete for its actuating coil |35 a holding circuit through a current limiting resistor |43 and a self-actuatedynormally open interlock contact |44. After the contactor B picks up it closes a circuit for an actuating coil |55 of thel time delay relay TDR thrugh the interlock contact 49 on the contactor B. Opening of the time delay relay contact |42 has no effectv for the present because it is now shunted by the lock-in contact |44 on the contacter B.

By way of summary then,'sirnple closure of one or the other of the dead man interlock contacts 53 or 54 immediately increases the engine speed to a predetermined minimum, picks up the brak.- ing contacter B to shunt the braking resistor 2|,

and completes a field excitation circuit for thev position of maximum forward torque, the vehi- .f

cle will begin to accelerate from standstill in the .forward direction. As the vehicle comes up to speed, the motor current falls off and the voltage rises from a point |30 on the I. R. line F of Fig. 5 and along a low engine speed power curve C to Some point such as the point |3|. It will be understood that as the line current diminishes, the motor torque also diminishes so that the point |3| is determined as that point at which the total motor torque just overcomes the resistance to motion of the vehicle.

With the establishment of a current in the generator output circuit, the directional winding |31 of the reverse power relay RPR is energized in proportion to the motor voltage and cumulatively with respect to the polarizing winding |36 of the reverse power relay. The winding |31 thus tends to hold the reverse power relay in its actuated position.

To increase the speed-of the vehicle the accelerator pedal 26 must be depressed. The engine |0 is therebyspeeded up and its power increased.

the motor torque. The vehicle will therefore speed up until the increased motor torque just balances the resistance of the terrain over which the vehicle is operating. It should be noted that thespeed of the vehicle cannot be increased simply by moving the steeringhandles forward (assuming that they had previously been removed from the maximum forward position) to increase the motor excitation. This will be evident from a consideration of the fact that no greater power is thereby supplied to the motor since the generator power output is not changed by moving the steering handles. 'I'he only effect of moving the steering handles forward simultaneous` ly is to increase the motor voltage and thus decrease the motor currentl so that the operating point moves along one of the constant horsepower curves such as that shown in Fig. to a higher voltage point.

If during forward motoring operation the accelerator pedal is depressed sufficiently far to increase the vehicle speed to its predetermined maximum value, the speed limit circuit functions in the manner heretofore described to diminish the excitation of the traction motors and to give a visual or audible indication at the overspeed indicating device |08. The operation of the speed limit circuit and its relation to the current limit feature has been explained hereinbefore. So far as the operator is concerned the functioning of the speed limit circuit and an indication on the overspeed device |08 should be taken as a signal for releasing the accelerator pedal to some extent, thereby to reduce the power output of the engine and to permit the system operating point to move off of the current limit. This is desirable as obviously it is more efficient to operate the system at low current and high voltage than to operate at low voltage at high current.

An outstanding feature of our arrangement is that, in the event that the vehicle while motoring forward encounters a downhill grade and the operator makes no attempt to check the vehicle speed, the speed limit protection will operate not only to reduce the engine horsepower output in the manner previously explained but will actually reverse the net excitation of the traction motor field windings thereby automatically to initiate dynamic braking operation and to maintain the vehicle at the maximum permissiblel speed. To illustrate, let it be assumed that the vehicle is brought to a steep downhill grade with the engine i3 running at its maximum speed and the steering levers 83 and 84 pushed forward to their maximum torque positions as shown at Fig. 1. As the vehicle speed increases the voltage of the tachometer generator 60 will become so great that the current spilled over from the tachometer generator through the battery 45 and the motor exciter differential windings 90 and @i will become sufficient completely to overpower the main control field windings T2 and 'i3 and to reverse the net excitation of the motor exciters i4 and l5 and the motors I5 and Il thereby to initiate dynamic braking operation.

The manner in which the vehicle may be steered by reducing cr reversing the torque on one or the other of the motors has been previously described. Also, dynamic braking without steering by simultaneous transfer of the steering handles 83 and 84 to their reverse quadrants has been explained with reference to the motor field excitation circuit. It may now be noted that when such dynamic braking operation is initiated the back electromotive force of the traction motors IE and il is reduced to zero 'and then re versed. As the back electromotive force of the traction motor approaches Zero or begins to reverse in going into braking, the directional coil |31 of the reverse power relay RPR functions to drop out the reverse power relay thereby to deenergize the actuating coil |35 of the braking contacter B and to cause the contacter B to drop out and unshunt the braking resistor 2li During dynamic braking operation the power generated by the generator |2 and the traction motors |6 and V.' is absorbed in the braking resistor. Proper operation of the reverse power relay RPR is insured -by a pair of capacitors |50 and |5| connected across a resistor |52 in the circuit of the directional coil |31. These capacitors function to cause the reverse power relay to anticipate its normal setting sufficiently so that even when the rate of change of motor voltage is greatest the braking contacter B is opened at the desired time. As braking proceeds with the motor voltage reversed the reverse power relay RPR remains dropped out and the braking contactor B likewise remains deenergized. As the motor voltage decreases with decreasing vehicle speed, the voltage of the main generator I2 increases to maintain the continuing unidirectional current in the main line circuit. Since the current in both the motor armatures and the motor field windings is maintained, dynamic braking may be made effective until the vehicle reaches standstill.

Opening of the braking contactor B indicating that dynamic braking is in progress also effects a number of protective functions necessitated by braking operation. In the first place the interlock contact 59 of the contactor B disables the current limit circuit thereby to remove the current limit effect during dynamic braking. This will be evident from Fig. 2, and is necessitated by the fact that the voltage of the engine driven generator I2 cannot be permitted to reverse under the influence of the differential exciter field winding 40 in an attempt to malntain the line current at the current limit value. If the generator voltage were permitted to re verse it would mean that the generator was acting as a motor to drive the internal combustion engine Ill. While slight overspeeding of the engine is permissible, any substantial overspeeding rnust be avoided in order to prevent damage to the engine. A second normally closed interlock |53 is also included on the braking contactor B for the same reason. When the braking contactor drops out the interlock |53 shunts the resistor 55 to insure that maximum forward field is maintained on the generator exciter i3. Unless the generator exciter forward field 4| is kept at its maximum value during braking, it is possible that even the minimum strength of the generator differential field 4| would be sufficient to cause the reversal of the main generator voltage and overspeeding of the engine |D.

It is also to be noted that during braking the interlock contact |38 on the contacter B is closed to shunt the holding resistor |39 in circuit with the reverse power relay polarizing coil |36 thereby to change the calibration of the reverse power relay and to restore its normal pickup circuit. The interlock Contact |4| on the braking con tactor B also closes during dynamic braking to short circuit the holding resistor |43 in series with the actuating coil |35 of the braking contactor and to restore the normal pickup calibration of the braking contacter.

During dynamic braking operation While the reverse power relay RPR remains dropped out energzation of the braking contacter coil |35 is precluded by the time delay relay TDR, the actuating winding of which is maintained energized through a normally closed interlock contact |56 on the reverse power relay RPR which shunts the now open interlock contact 49 on the contactor B. When dynamic braking is completed and lthe vehicle speed is reduced to zero,

the RPR relay recognizes this fact by the fact that the motor voltage is substantially zero. The reverse power relay then picks up and deenergizes the forward field winding 4l of the generator exciter I3 at the RPR interlock contact |50. Opening of the :ontact |56 also deenersizes the time delay relay TDR. 'I'he time delay relay, however, does not immediately drop out. Thus for a short time the generator exciter I3 is energized only by its diil'erential eld winding 40 and its voltage is rapidly brought to zero. After a suitable time which is just suiiicient to allow the line current to come approximately to zero, the time delay relay TDR drops out and `closes its interlock contact |42. Closure of the contact |42 completes an energizing circuit for the braking contactor B and contactor B picks up and locks itself in through its interlock contact |44. It will be understood, of course, that at this time the operator will release the dead man handle interlocks 53 and 54 and return the steering handles 83 and 84 to their biased normal position. If the operator retains the handles in the braking position, the reenerglzation of the braking contactor B will complete an energizing circuit for the generator forward field winding through its interlock contact 49 and initiate reverse motoring operation.

Referring now to Fig. 4 the purpose of a blocking rectiiier |60, not previously mentioned, will become evident. The rectifier |60 is located between the points 80 and 8| of the potentiometer bridges 14 and 15 and ground, and has for its purpose the prevention of a reverse current in the mainline circuit. Let us assume, for example, that the steering handles 83 and 84 are moved to braking position with the dead man handle interlocks 53 and 54 open. Under these conditions the forward field 4| of the generator exciter I3 would be disabled so that there would be nothing to determine the direction of current iiow in the generator output circuit. A reverse line current can start in the system, and under such conditions the reverse power relay RPR would pick up under the influence of the windving |31 alone and cause closure of the braking contacter B. Under such conditions the traction motors I8 and I1 would build up as series generators as desired but without the braking resistor 2| in the main line circuit, so that all the power would be absorbed in the generator l2 thereby causing the generator to overspeed the engine I0. The blocking rectifier |60 serves to prevent the motors from receiving any excitation due to a reversed line current, so that braking under'such conditions is not possible. Thus at Fig. 4 it will be evident that if the potential of the grounded wire exceeds the potential ofy the point 8|, as due to a reversal of line current, the` potentiometers 14 and 15 will be short circuited through the blocking rectifier |60.

From the foregoing it will be clear that we have provided an extremely flexible electric power system and motor control system of general application which, when used in its entirety, possesses particularly attractive features for application to a track laying vehicle. The system is remarkable for its speed of response and for its absolute protection against overload without regard to the severity of requirements imposed upon it by the operator. These virtues, and particularly the speed of response, are due primarily to the refined limiting and control circuits applied in conjunction with an electric power system While we have described only one preferred embodiment of our invention by way of illustration it will be evident that many modifications will occur to those skilled in the art and that we, therefore, intend by the appended claims to cover .all such modifications as fall within the true spirit and scope of our invention.

What We claim as new and desire to secure by Letters Patent of the United States is:

l. An electric power system including a direct current generator having an output circuit, a direct current motor having its armatureconnected ln said output circuit, said generator having a voltage current characteristic at any predetermined speed bounded by maximum current limit and maximum available horsepower input in said generator, means for automatically and continuously maintaining in said circuit a directly controlled unidirectional current within the limits of said characteristic, a eld excit- -ing winding for said motor, a source of direct current supply for energizing said ileld exciting winding, manually operable selector means for varying themagnitude and direction of ener- 'gization of said eld exciting winding thereby to control the speed, torque, and direction of rotation of said motor, and means responsive to the speed of said motor and operable to reduce the excitation of said motor when said motor attains a predetermined speed thereby to limit said motor to said predetermined speed,

2. A power system for a self-propelled electric vehicle including a prime mover, a direct current generator driven by said prime mover and having an output circuit, a direct current motor having its armature permanently connected in said output circuit, said generator being arranged continuously to maintain in said'output circuit a circulating unidirectional current, means 'for limiting said current to a predetermined maxlmum value independently of the effective resistance of said output circuit, a separately excited field winding for said motor, a source'of direct current supply for energizing said field winding, manually operable means for controlling the magnitude and direction of energization of said eld winding to determine the speed, torque, and direction of rotation of said motor, and means for reducing the excitation and power of said motor when said vehicle tends to exceed a predetermined speed.l

3. A power system for a self-propelled electric vehicle including a prime mover, a direct current generator arranged to be driven by said prime mover and to supply current to an output circuit, a direct current motor having its armature connected in said output circuit, said generator being arranged continuously to maintain in said circuit a circulating unidirectional current, means responsive to said current to limit said current to a selected maximum value inde- ,pendently of the effective resistance of said outfield winding in accordance with said current' thereby to impart to said motor a predominantly series type speed-torque characteristic, and means for reducing the excitation of said motor in accordance with the speed of said vehicle to exaggerate said series characteristic.

4. A power system for a self-propelled electric vehicle, including a prime mover, a direct current generator arranged to be driven by said prime mover and to supply electric current to an output circuit, a plurality of direct current motors having their armatures connected in series circuit relation in said output circuit, said generator being arranged continuously to maintain in said output circuit a circulating unidirectional current, means responsive to said current to limit said current to a selected maximum value independently of the effective resistance of said output circuit, means for selecting said maximum value in accordance with the speed of said vehicle, a separately excited field winding` for each of said motors, means for energizing each of said windings in accordance with the current of said output circuit thereby to impart to said motors a predominantly series type speedtorque characteristic, manually operable means for independently selecting the standard of energization of each of said field windings and for independently reversing the energization thereof separately to control the speed, torque, and direction of rotation of said motors, and means for reducing the excitation of said motors in accordance with the speed of said vehicle to exaggerate said series characteristic;

5. A power system for a self-propelled electric vehicle comprising an internal combustion engine, a direct current generator arranged to be driven by said engine and to supply current to an output circuit, a direct current motor having its armature connected in said output circuit, said generatorV being arranged continuously to maintain in said circuit a circulating unidirectional current, means responsive to said current to limit said current in a selected maximum value independently of the effective resistance of said output circuit, means for selecting said maxi` mum value in accordance with the speed of said vehicle, a field exciting winding for said motor. dynamo-electric means including a control field winding energized in accordance with the current in said output circuit to supply to said exciting winding a current proportional to the current of said output circuit thereby to impart to said motor a predominantly series type speedtorque characteristic, manually operable means for selecting the standard of energization of said control field winding thereby to control the speed, torque, and direction of rotation of said motor, and means including a differential control field winding for said dynamo-electric means for reducing the energization of said field exciting winding in accordance with the speed of said vehicle thereby to exaggerate said series characteristic.

6. A power system for a self-propelled electric vehicle comprising an internal combustion engine, a direct current generator arranged to be driven by said engine and to supply current to an output circuit, a direct current motor having its armature connected in said outputY circuit, said generator being arranged continuously to maintain in said circuit a circulating unidirectional current, means responsive to said current to limit said current to a selected maximum value independently of the effective resistance of said output circuit, means for selecting said maximum current value in accordance with the speed of said vehicle, a field exciting winding for said motor, dynamo-electric means including a control field winding energized in accordance with the current in said output circuit to supply to said exciting winding a current proportional to the current of saidoutput circuit thereby to impart to said motor a predominantly series type speed-torque characteristic, manually operable means for selecting the standard of energization of said control field winding thereby to control the speed, torque, and direction of rotation of said motor, means including a differential control field winding for said dynamo-electric means for reducing the energization of said field exciting winding in accordance with the speed of said vehicle thereby to exaggerate said series characteristic, and means for limiting the speed of said vehicle to a predetermined maximum value comprising supplementary excitation means for said difierential control field winding operable upon attainment by said vehicle of said predetermined speed.

7. A power system for a self-propelled electric vehicle having laterally spaced driving elements comprising an internal combustion engine, a direct current generator connected to be driven by said engine and arranged to supply current to an output circuit, a plurality of direct current motors connected to drive said laterally spaced driving elements and having their armatures permanently connected in series circuit relation in said output circuit, excitation means for said generator arranged continuously to maintain in said output circuit a directly controlled unidirectional current during accelerating and electric braking operation of said motors, said excitation means including means for reducing the excitation of said generator in accordance with the maximum available horsepower output of said engine, means for limiting said unidirectional current to a predetermined maximum permissible value regardless oi' the effective resistance of said motor armatures, a separately excited field exciting winding for each of said motors. separate dynamo-electric means driven by said engine for energizing each of said field excitingr windings, a control field winding for each of said dynamo-electric means, a source of direct current supply for energizing each of said control v field windings, and a potentiometer bridge interposed between said source of direct current supply and each of said control held windings for selecting the direction and magnitude of excita- 'tion of each of said motors independently of the excitation of any other of said motors and of said unidirectional output circ-.lit current.

8. A power system for a self-propelled electric vehicle having laterally spaced driving elements comprising an internal combustion engine, a direct current generator connected to be driven by said engine and arranged to supply current to -an output circuit, a plurality of direct current motors connected to drive said laterally spaced ldriving elements and having their armatures permanently connected in series circuit relation in said output circuit, excitation means for said generator arranged continuously to maintain in said output circuit a directly controlled unidirectional current during accelerating and braking operation of said motors, said excitation means including means for reducing the excitation of said generator in accordance with the maximum available horsepower output of said engine, means for limiting said unidirectional current to a predetermined maximum permissible value regardless of the eilective resistance of said motor armatures, a separately excited field exciting winding for each of said motors, separate dynamo-electric means driven by said engine for energizing each of said field exciting windings; a

asoaals control field winding for each cf said dynamoelectric means, each of said control field windings being connected across a portion of said output circuit thereby to impart to said motors a predominantly series type speed-torque characteristic, a potentiometer bridge interposed between said output circuit and each of said control field windings for selecting the direction and magnitude of excitation of each of said motors independently of the excitation of any other of said motors and of said unidirectional output circuit current, a differential control field winding for each of said dynamo-electric means, and means for energizing said differential control field windings in accordance with the speed of said vehicle toexaggerate said series characteristic.

9. A generator excitation system comprising a direct current generator having an output circuit, means for exciting said generator in substantially inverse proportion to the current fiowing in said output circuit, said means comprising a current responsive elemet and a resistor connected in series circuit relation lacross a portion of said output circuit, and means for limiting the current in'said output circuit to a predetermined maximum value comprising a source of bias po tential connected through unidirectionally conducting means in parallel circuit relation with said resistor to oppose the voltage drop thereacrossywhereby when the current through said resistor exceeds a predetermined value said resistor is partially shunted through said source of bias potential thereby to increase the energization of said current responsive element and to decrease the excitation of said generator.

10.v A generator excitation system comprising ,a direct current generator having an output circuit, a main field winding for said generator, differential means for controlling the excitation of said generator in accordance with the current in said output circuit, said differential means comprising an auxiliary field exciting winding connected in series circuit relation with a resistor across a portion of said output circuit, and means for limiting the current in said output circuit to a predetermined maximum value comprising a source of substantially constant bias potential connected through a polarity responsive element across said resistor in opposing relation to the voltage drop thereacross, whereby said resistor is partially shunted through said source of constant bias potential and the energization of said auxiliary field exciting winding increased when the voltage across said resistor exceeds sai.: bias potential.

11. A generator excitation system comprising.

a direct current generator having an output circuit, a main field winding for said generator, an auxiliary dynamo-electric machine connected to supply excitation current to said main field winding, main and differential control field windings for said machine, said differential control field winding being connected in series circuit relation with a. current limiting resistor across a portion of said output circuit, and means for limiting the current in said output circuit to a predetermined maximum value comprising a source of substantially constant bias potential connected through a unidirectionally conducting element in parallelopposing relation with respect to said current limiting resistor, whereby said resistor is partially shunted through said source of bias potential when the voltage drop across said resistor attains a value suiiicient to cause conduccitation means for said generator, means responsive to the current in said output circuit and operatively associated with said excitation means for limiting said current to a selected maximum value, and means responsive to the speed of said motor for modifying the control of said current responsive means to select said maximum value in accordance with the speed of said motor.

13. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a direct current motor having its armature connected in said output circuit, excitation means for said generator, means responsive to the current in said output circuit for reducing the excitation of said generator to limit said current to a predetermined maximum value, and means responsive to the speed ofA said motor for modifying the operation of said current responsive means to vary said predetermined maximum current value inversely as the speed of said motor.

14. A motor control system comprising a direct current generator connected to supply current to an output circuit, a, direct current motor having its armature connected in said output circuit, excitation means for said generator, differential means for controlling the excitation of said generator in accordance with the current in said output circuit, said differential means comprising a, current responsive element and a resistor connected in series circuit relation across a portion of said output circuit, and means for limiting the l current in said output circuit to a maximum value selected in accordance with the speed of said motor comprising a unidirectionally conducting shunt circuit for said resistor including a source of bias potential having a magnitude inversely proportional to the speed of said motor and disposed in said. shunt circuit to oppose the voltage drop across said resistor.

15. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a direct current motor having its armature connected in said output circuit, a

main field exciting winding for said generator,

an auxiliary dynamo-electric machine connected to supply excitation current to said field exciting winding, means yfor reducing the excitation of said generator in accordance with the current in said output circuit comprising a differential field exciting winding for said dynamo-electric machine connected in series circuit relation with a current limiting resistor across a portion of said output circuit, means for limiting the current in said output circuit to a selected maximum value comprising a source of substantially constant bias potential connected in a unidirectionally conducting shunt circuit across said resistor to oppose the voltage drop across said resistor, and means for reducing the current limit of said out- -lput circuit as the speed of said motor increases -to the voltage drop across said resistor thereby to increase the net voltage opposing said source of constant bias potential as the speed of said motor increases.

.dynamo-electric machine connected to suppl) excitation current to said field exciting winding, a main and differential control field windings for said dynamo-electric machine, said differential control field winding being connected in series circuit relation with a current limiting resistor across a portionl of said output circuit thereby to control the excitation of said direct current generator in accordance with the current in said output circuit, means for limiting the current in said output circuit to a selected maximum value comprising an opposing bias potential of substantially constant value connected in a unidirectionally conducting shunt circuit across said current limiting resistor to oppose the voltage drop thereacross, and means for reducing the current limit of said output circuit in accordance with the speed of said vehicle comprising a tachometer generator connected to be driven by said traction motor and arranged to supply a bias potential proportional to the speed of said motor to diagonally opposite terminals of a rectiiier bridge, the direct current terminals of said rectiiier bridge being connected in said unidirectionally conducting shunt circuit in opposing rela` tion to said constant bias potential thereby to reduce the net bias potential in said shunt circuit directly as the speed of said motor increases.

17. A power system for a self-propelled vehicle comprising an internal combustion engine, a direct current generator connected to be driiren by said engine and arranged to supply current to an'output circuit, a plurality of direct current traction motors having their armatures connected in series circuit relation in`said output circuit, separately excited individually controllable field` excitation means for each of said motors, and means for limiting the current in said output circuit to a maximum value selected in accordance with the speed of said vehicle independently of the effective resistance of said motor armatures comprising a main field exciting winding for said generator, an auxiliary dynamo-electric machine connected to supply excitation current to said field exciting winding,

' said machine having main and differential control eld windings, said differential control field winding being connected in series circuit relation with a current limiting resistor across a portion of said output circuit to control the excitation of said generator in accordance with the current in said output circuit, a source of substantially constant bias potential connected in a unidirectionally conducting shunt circuit across said current limiting resistor to oppose the voltage drop thereacross, said shunt circuit including a rectier bridge, and a tachorneter generator connected to be driven by one of said motors and arranged to supply to the alternating current terminals of said rectifier bridge a bias potential proportional to the speed of said one motor. the direct current terminals of said bridge being connected in said shunt circuit to oppose said constant bias potential thereby to reduce the net potential opposing the voltage drop across said .resistor as the speed of said vehicle increases.

18. A power system for a self-propelled electric vehicle comprising an internal combustion engine, a, direct current generator connected to be driven by said engine and arranged to supply current to an output circuit, a plurality of direct said vehicle independently of the effective re-v sistance of said motor armatures.

19. A power ,system for a. self-propelled electric vehicle comprising an internal combustion engine, a direct current generator connected to be driven by said engine and arranged to supply current to an output circuit, a direct current traction motor having its armature connected in said output clrcuit,`excitation means for said generator arranged continuously to maintain in said output circuit a unidirectional circulating current during both motoring and dynamic braking operation of said motor, means responsive to the current in said output circuit for controlling the excitation of said generator to limit said current to a predetermined maximum value during motoring operation, reversible field excitation means for said motor arranged to select the direction and magnitude of motor torque thereby to control the motoring and dynamic braking operation of said motor, and means responsive to the polarity of motor voltage for disabling said current limiting means during dynamic braking operation thereby to prevent motoring operation of said generator and overspeeding of said engine.

20. A power system comprising an internalY combustion engine, a direct current generator driven by said engine and arranged to supply power to an output circuit, a direct current motor having its armature connected in said output circuit to receive power from said generator during motoring operation, means operable in conjunction with said generator during motoring operation to limit the current in said output circuit to a predetermined maximum value independently of the effective resistance of said motor armature, means for connecting said motor to supply power to said output circuit for dynamic braking operation, and means operable approximately as the motor power passes through zero to disable said current limiting means thereby to minimize motoring operation of said generator and overspeeding of said engine.

21. A -power system comprising an internal combustion engine, a direct current generator i driven by said engine and arranged to supply power to an output circuit, a direct current motor having its armature connected in said output circuit to receive power from said generator during motoring operation, means operable in oo njunction with said generator during motoring operation to limit the current in said output circuit to a selected maximum value independently of the effective resistance of said motor armature,

lmeans responsive to the speed of said motor to :,soaois Y vehlriclel comprising an internal combustion enginefa vdirect currentgenerator connected to be y driven by said engine and arranged to supply current to an output. circuit, a direct current traction. motor having its armature permanently connected in said output circuit, excitation means for said generator arranged continuously to maintain in said output circuit a unidirectional circulating current during both motoring and dynamic braking operation of said motor, means responsive to the current in said output circuit for controlling the excitation of said generator.

to limit said current to a predetermined maximumvalue during motoring operation, energy absorbing means permanently connected in said control motoring and dynamic braking operation of said motor, means for limiting the current in said output circuit to a predetermined maximum value comprising av unldirectionallyconducting shunt circuit for said resistor including a source of bias potential connected to oppose the voltage drop across said resistor, and means responsive to the polarity of the motor armature yvoltage for disabling said shunt circuit thereby to remove vsaid predetermined current limit during 4dynamic braking operation, whereby motoring of said generator and overspeeding of said engineV during braking operation are minimized.

output circuit, shunting means for said energy r iting means approximately as the motor voltage passes through zero to initiate dynamic braking operation.

23. A power system for a sell-propelled electric vehicle comprising an internal combustion engine, a direct current generator connected to be driven by said engine, and arranged to supply current to an output circuit, a direct current traction motor having its armature connected inv said output circuit, excitation means for said generator arranged continuously to maintain in said output circuit a unidirectional circulating current during both motoring and dynamic braking operation of said motor. means responsive to ythe current in said output circuit for controlling the excitation of said generator to limit said current to a predetermined maximum value during motoring operation independently of the ei'- fective resistance of said motor armature, reversible neld excitation means for said motor arranged to vary the direction andgmagnitude oi' motor .torque thereby to control motoring and dynamic braking operation o! said vehicle, a dynamic braking resistor permanently connected in said output circuit, shunting means for said dynamic braking resistor, and a polarized relay responsive to motor voltage for actuating said lshunting means to disable said dynamic braking resistor during motoring operation and for unshunting said resistor and disabling said current limiting means approximately as the motor voltnamic braking koperation of said motor. saidexcitationmeans comprising a neld winding. connected in series circuit relationwitha current Alimiting resistnracrossaportionolsaidoutput reversible .excitation means for said moselectively to determine the magni-- crmom torque thereby to' 25. A motor control system comprising a direct current generator connected to supply current to .an outputcircuit, a direct current motor having its armature connected in said output circuit. means for exciting said generator continuously `to maintain in said output circuit a unidirectional .'means during motoring operation and for dis- I abling said shunting means and said current limcurrent during both motoring and dynamic braking operation of said motor, diilerential means forreducing the excitation oisald generator in accordance with said current, said diilerential means being energized across an inductive portion oi' said output circuit, means for reversing the excitation of said motor to supply power to said output circuit for dynamic braking operation thereby to produce a rapid increase in th'e magnitude of said unidirectional current and to establish across said inductive portion oi said circuit an inductive voltage, said inductive voltage transiently increasing the energization oh said ing its armature connected in said output circuit,

an auxiliary dynamo-electric machine for exciting said generator continuously to maintain in said output circuit a unidirectional current during both motoring and dynamic braking operation oi' said motor, said auxiliary dynamo-electric machine including main and differential control neld windings, said differential control neld winding being energized across an inductive portion of said output circuit to reduce the excitation of said generator in accordance with said current, manuallycontrollable reversible excitation means for said motor arranged to select the direction and magnitude of motor torque, thereby to control motoring and dynamic braking operation of said motor, said motor upon reversal o! the excitation supplying power to said output'circuit and producing a rapid increase in` said circulating current so that the inductive voltage appearing across said diii'erential control neld winding aids in reducing the excitation of said generator and restoring said circulating current to its steadystate value.

' 27. In a motor control system, a direct current motor having an armature and ai'ield winding, a direct current generator having a main field winding, an output circuit for said generator -including said 'motor amature and a dynamic.

braking resistor, switching means for slumting said resistor during motoring operation-reversible means for said motorneld winding Y fllmblitoenect motoring and dynamic braki ing. otsaid motor. said switching ibeing'op'erable upon initiation ci dynamic -brakmg l'un'srmnt and resistor. means controlled bysaidswitching means ior'energinng said generator neld winding. and time element ing means to deenergize said generator tieid winding for a. predetermined interval upon the cessation of dynamic braking operation thereby to hasten the decrease of generator current and voltage.

28. In a motor control system, a direct current motor having an armature and a eld winding, a direct current generator having a main field exciting winding, an output circuit for said generator including said motor armature and a dynamic braking resistor, switching means for shunting said resistor during motoring operation oi said motor, reversible energizing means for said motor field winding selectabiy to effect motoring and dynamic braking operation of said motor, relay means responsive to the polarity of motor armature voltage for controlling said switching means, said relay means being operable upon initiation of dynamic braking operation to disable said switching means thereby to unshunt said resistor, means controlled jointly by said relay means and said switching means for con-l trolling the energization of said generator iield winding, and time-delay means operable in conjunction with said relay means and said switching means temporarily to disable said generator iieid winding upon the cessation of dynamic braking operation thereby to hasten the decrease of generator voltage and current.

29. In a motor control system, a direct current driving motor including an armature and a field winding, means for connecting said armature to a source of electric current supply, excitationmeans for supplying to said eld winding an excitation current substantially proportional to the current in said armature thereby to impart to said driving motor a predominantly series speedtorque characteristic, and means responsive to the speed of sai-:i motor for modifying the output oi said excitation means to exaggerate said series characteristic.

3Q. In a motor control system, a direct current driving motor including an armature and a eld winding, means for connecting said armature to a source of electric current supply, means including a dynamo-electric machine for supplying excitation current to said field winding, means responsive to the current in said armature for exciting said dynamo-electric machine in accordance with said current thereby to impart to said driving motor a predominantly series speedtorque and characteristic, and means responsive to the speed of said motor for modifying the autput of said dynamo-electric machine to exaggerate said series characteristic.

31. In an electric motor control system a direct current driving motor including an armature and a field winding, means for connecting said armature to a source of electric current supply, means for supplying excitation current to said eld winding including an auxiliary dynamo-electric machine having a control iield winding, means responsive to the current in said armature for energizing said control field winding in accordance with said current thereby to impart to said driving motor a predominantly series speed-torque characteristic, and means for exaggerating said series speed-torque characteristic comprising a second control eld winding for said dynamoelectric machine, and means responsive to the speed of said mo'tor for energizing said second control field winding in opposition to said first control field winding.

32. In an electric motor control system, a direct current driving motor including an armature and aseaeis a field winding. means for connecting said armature to a source of electric current supply, means operable in conjunction with said source of supply to limit the current in said armature to a predetermined maximum value independently of the effective resistance of said armature, means for supplying excitation current to said iield winding including an auxiliary dynamo-electric ma- .chine having a control iield winding, means responsive to the current in said armature for energizing said control iield Winding in accordance with said current thereby to impart to" said driving motor a predominantly series speed-torque characteristic, and means for exaggerating said speed-torque characteristic comprising a difierential control eld winding for said dynamo-electric machine energized in accordance with the speed of said motor, a resistance oridge having output terminals connected to said differential control eid winding and input terminals connected to a second source of electric current supply having a voltage proportional to the speed of said motorf one arm of said bridge comprising a negative impedance-voltage characteristic impedance to limit the energization of said diffe"- ential control iield winding to a predetermined maximum value and thereby to preclude increase of said armature current to said current limit value under normal operating conditions.

33. In a motor control system, a direct current driving motor including an armature and a field winding, means for connecting said armature to a source of electric current supply, excitation means for supplying to said field'winding an excitation current substantialiy proportional to the current in said armature thereby to impart to said driving motor a predominant series type speedtorque characteristic, and means responsive to a motor armature current of predetermined value 40 for limiting the output of said excitation means to a predetermined maximum value for all armature currents above said predetermined value thereby to impart to said driving motor a pre- ;lotninantly shunt type speed-torque character- 34. In a motor control system, a direct current driving motor including an armature and a iield winding, means for connecting said armature to a source of electric current supply, means including a dynamo-electric machine for supplying excitation current to said ileld winding, means responsive to the current in said armature for normally controlling the output of said dynamoelectric machine in accordance with said current thereby to impart to said driving motor a predominantly series type speed-torque characteristie, and means responsive to a motor armature current of predetermined value for limiting the energization of said current responsive means to a predetermined maximum value for all armature currents above said predetermined value thereby to impart to said driving motor a predominantly shunt type speed-torque characteristic.

35. A motor control system for an electrically driven vehicle comprising a, direct current generator arranged to supply current to an output circuit, a direct current traction motor having its armature connected in'said output circuit, excitation means foz-said generator arranged continuously to maintain in said output circuit a unidirectional circulating current during both motoring and electric braking operation of said motor, a iield exciting winding for said motor, means connected across a portion of said output circuit for energizing said field exciting winding 

